Microbending will occur in an optical fibre which is subjected to an external force, with subsequent light attenuation. Although this is a disadvantage in signal transmission, microbending can be utilized in a manner which will enable the fibre to be used as a detector or sensor element. An example of one such force detecting element is found described in European Patent Specification Number 0188512. An optical fibre having a core and cladding is embraced by a primary cover, and a twisted tape is disposed between the cladding and the primary cover. The twisted tape is responsible for the microbending when a force is exerted on the fibre.
A fibre cable for detecting temperature is illustrated in Japanese Patent Application No. 59-50676. The optical-fibre cladding has wound therearound a wire consisting of memory-metal, which contracts when exposed to a given detection temperature and therewith exerts pressure on the cladding and causes microbending of the fibre. Each molecule in the fibre core reflects light by Rayleigh-scattering and the intensity of the light reflected is directly proportional to the intensity of the passing light pulse. The intensity of the reflected light pulse decreases as a result of microbending of the fibre. The reflected light pulse can be detected and the position at which microbending occurs along the fibre can be established by so-called OTDR, Optical Time Domain Reflectometry. The drawback with this device is that in many applications it is difficult to obtain a memory metal which will produce microbends of desired sizes and at desired temperatures. The device is also relatively complicated to manufacture.